Oracle Real Application Cluster (RAC) is a well-known product among Oracle’s solutions to maintain high availability of your business data. Oracle RAC allows the work load to be shared among all the cluster nodes, with N-1 tolerance configuration in case of node failures, where N is the total number of nodes. Oracle RAC is constantly improving on every version and this time was not different. The new version incorporates two properties called “Flex ASM” and “Flex Cluster” that gives support to the demand requirements on Cloud Computing oriented environments.

Oracle RAC 12c introduces two new concepts:

Hub Nodes: They are connected among them via private network and have direct access to the shared storage just like previous versions. These nodes are the ones that access the Oracle Cluster Registry (OCR) and Voiting Disk (VD) directly.

Leaf Nodes: These nodes are lighter and are not connected among them, neither access the shared storage like the Hub Nodes. Each Leaf Node communicates with the Hub Node that is attached to, and its connected to the cluster via the Hub Node that is linked to.

This topology allows loosely coupled application servers to form a cluster with tightly coupled database servers. Tightly coupled servers are Hub Servers that share storage for database, OCR and Voting devices as well as peer-to-peer communication with other Hub Servers in the cluster. A loosely coupled server is a Leaf Server that has a loose communication association with a single Hub Server in the cluster and does not require shared storage nor peer-to-peer communication with other Hub or Leaf Servers in the cluster, except to communicate with the Hub to which it is associated. In, Leaf Servers are designed for greater application high availability and multi-tier resource management.

Prior to Oracle 12c, for a database instance to use ASM it is expected that the ASM instance must be up and running on all nodes before the database instance is brought up. Failure of ASM instance to come-up means that database instance using ASM at the storage level cannot be brought up. This literally means that the database instance is not accessible immaterial of the technologies put in use i.e. RAC, ASM and Shared Storage.

With the launch of Oracle 12c the above constraint has been addressed with the feature called Oracle Flex ASM which primarily has a feature to fail over to another node in the cluster. Essentially a Hub and Leaf architecture, the connection of a failed node is seamlessly transferred to another participating node by way of a replacement ASM instance by Oracle Clusterware. The number of ASM instances running in a given cluster is called ASM carnality with a default value of 3. However the carnality value can be amended using the Clusterware command.

Implementation aspects of Oracle Flex ASM

The Oracle Flex ASM can be implemented in two ways:

  • Pure 12c Flex ASM (Same Version)
    • Both Grid Infrasctructure (GI) and Database running on Oracle 12c
  • Pre Oracle 12c Mixed (Different Versions)
    • As normal ASM instance will run on each node with Flex configuration to support pre 12c database. The compatibility parameter of ASM disk group is used to manage the compatibility of between and among database instances. Advantage of this approach is that if an Oracle 12c database instance loses connectivity with an ASM instance, then the database connections will failover to another ASM instance on a different server. This failover is achieved by setting the carnality to all.

Converting Standard Cluster to Flex Cluster

  1. Main requirement for flex cluster requires GNS. If a GNS doesn’t exists setup GNS and add the GNS VIP to cluster.

2. Now convert the standard cluster mode to flex. This requires a cluster downtime.To change the cluster mode to flex as root run the following .

Repeat the above steps to all other nodes

Hope you like this article. In next article we will do conversion of Hub Node to Leaf Node in Flex Cluster.

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